| Parenchyma is an important component of secondary xylem of woody angiosperms,and its stored water and nonstructural carbohydrates play an important role in the maintenance and repair of xylem hydraulic functions.Since the parenchyma fraction of xylem is closely related to its material storage capacity,investigating the variability of parenchyma fraction and how it coordinates with the hydraulic function of xylem can contribute to a better understanding of the physiological mechanisms by which plants adapt to their environment and respond to climate change.It has been shown that parenchyma fraction varies greatly among angiosperms,but the source of variation and its relationship with vessel size,are unclear.For this purpose,156 individual trunk samples of 45 common woody angiosperms were collected along an elevation gradient from 600 m to 1600 m in Anhui Tianma National Nature Reserve,and their xylem anatomical characteristics were quantified.The relationship between parenchyma fraction and hydrothermal conditions and individual size was analyzed,and the inter-and intra-species variation of parenchyma fraction was quantitatively decomposed,and the relationship between parenchyma fraction and vessel size was analyzed,and the significance of parenchyma fraction variation for plant ecological adaptation was discussed.The main findings are as follows:(1)At the individual level,the total parenchyma fraction(sum of axial and ray parenchyma fraction)ranged from 4.4% to 46.8%,with axial parenchyma fraction ranging from 0.2% to 25.9% and ray parenchyma fraction ranging from 3.9% to 31.1%.In view of the important role of vasicentric tracheids in promoting the efficiency and safety of xylem hydraulic transport,we further analyzed species with and without vasicentric tracheids separately and found that interspecific variation in parenchyma fraction was usually greater than intraspecific variation,but intraspecific variation in ray parenchyma fraction was greater than interspecific variation in tree species with tracheids.The variation in parenchyma fraction was mainly influenced by temperature and tree height.The higher the mean annual temperature,the higher the parenchyma fraction,and the higher the individual height,the lower the parenchyma fraction.But after controlling for species differences,there was no significant relationship between mean annual temperature and parenchyma fraction;for species without tracheids,tree height was significantly negatively correlated with both axial parenchyma fraction and total parenchyma fraction(P < 0.05).(2)The relationship between parenchyma fraction(i.e.,vessel-to-parenchyma connectivity)in the 10 μm range around the vessels and environmental factors and tree height was further analyzed at the individual level.Vessel-to-total parenchyma connectivity ranged from 5.9% to 90.1%,vessel-to-axial parenchyma connectivity ranged from 0.5% to 58.8%,and vessel-to-ray parenchyma connectivity ranged from 2.3% to32.7%.Interspecific variation in vessel-to-parenchyma connectivity was generally greater than intraspecific variation,but intraspecific variation in vessel-to-total parenchyma connectivity and vessel-to-ray parenchyma connectivity was greater than interspecific variation in species with tracheids.The vessel-to-parenchyma connectivity was not significantly correlated with environmental factors and tree height,regardless of whether the effect of species differences was controlled.(3)At the individual level,there was a significant negative correlation(P < 0.01)between ray parenchyma fraction and vessel size in species without tracheids and no significant correlation between parenchyma fraction and vessel size in species with tracheids.Significant positive and negative correlations were found between vessel-toparenchyma connectivity and vessel size for species without and with tracheids,respectively.Within individuals,among the individuals with a significant correlation between single vessel diameter and vessel-to-parenchyma connectivity,more than 60% of them had a significant positive correlation.In summary,secondary xylem parenchyma fraction in woody angiosperms is mainly influenced by species and tree height and is not directly affected by hydrothermal conditions.The intensity of segmentation of wood space by ray parenchyma and the vessel-to-parenchyma connectivity affected vessel size,but both effects differed in species with and without tracheids.The findings contribute to both a better understanding of the role of parenchyma in the ecological adaptation strategies of woody plants and their role in long-distance water transport in the xylem,and to predicting the response of forest ecosystems to climate change. |
